Our laboratory is studying B cell differentiation to antibody secretion with two objectives in mind: to define the intracellular activation events and determine the control mechanisms involved. The strategies for achieving these objectives are based on findings that the J chain plays a critical role in the initiation and regulation of the pentamer IgM response. 1. Studies of the antigen signaling system have shown that antigen binding to the precursor B cell causes de novo transcription of the J chain gene. The mechanism of activation will be investigated by means of recombinant DNA technology. A J chain cDNA clone will be used to detect any rearrangement of the J chain gene during B cell differentiation and a genomic clone will be used to analyze the promoter region on the J chain gene and the requirements for in vitro transcription. In addition, the methods of somatic cell fusion and selective enrichment of RNA already developed in the J chain studies will be applied to identify other intermediates in the activation pathway. 2. Studies of in vivo pentamer assembly have shown that J chain synthesis is precisely controlled to optimize the polymerization of IgM. The regulatory mechanisms involved will be investigated by the use of three different approaches. (a) Linkage between the J chain gene and the heavy or light chain cistrons will be examined by heteroduplex mapping of J chain DNA and its flanking sequences. (b) The structure of nuclear and cytoplasmic J chain RNA will be determined to locate the J chain structural information and assess excised of untranslated sequences for regulatory function. (c) J and micron chain synthesis will be assayed in memory cells to clarify the relationship between the expression of J chain and the expression of the two forms of micron chain, one destined for membrane incorporation and the other for secretion.